A typical bypass-type differential pressure regulator is used to maintain a constant pressure drop across the inlet and outlet of a metering valve forming a part of a fluid pressure system. Such a typical regulator comprises a piston-like valve member adapted to reciprocate within a valve housing having high and low pressure ports communicating with high and low pressure sides of the system. The valve housing also includes a bypass port for bypassing pressure fluid from the high pressure side of the system to the bypass line. High pressure fluid admitted into the housing biases the valve member to a position tending to open the bypass port, while the low pressure fluid admitted into the housing biases the valve member to a position tending to close the bypass port. Typically, a spring coacts with the low pressure fluid to bias the valve member to the closed position. According to this arrangement, an increase in the high pressure, or a decrease in the low pressure, causes the valve member to shift to a position wherein it bypasses more flow from the high pressure side of the system to the bypass line, thereby maintaining a substantially constant pressure drop across the high and low pressure sides of the system. Conversely, a decrease in the high pressure or an increase in the low pressures results in the valve member being shifted to bypass less flow and thus maintain the substantially constant pressure drop.
Such prior regulators, however, often encounter difficulty in reliably maintaining a constant pressure drop over a wide range of flow rates. In particular, an undesirable increase in the pressure drop across the metering valve generally occurs at high bypass flows. As the bypass flow increases and the valve member opens, the biasing force exerted by the spring becomes progressively higher. As a result, the valve member encounters progressively greater resistance against movement to an open position. Also, increasing fluid reaction forces further resist opening of the valve member at high flows. As a result, the pressure drop across the system tends to increase at high bypass flows rather than remaining at a desired constant value.
One method of dealing with this problem can be found in U.S. Pat. No. 4,458,713 to Wernberg. In that patent, an additional force tending to close the valve was included along with the low pressure, the spring force, and the fluid reaction forces. This additional force, was designed to decrease at high bypass flows to compensate for the increased closing force caused by the spring and the fluid reaction forces at high bypass flows. In that patent, an orifice is created in the piston face in the high pressure chamber. This orifice communicates with an internal passage in the valve member. The bypass flow is through the orifice and passage, which communicates with the bypass port. As the bypass flow increases, the orifice introduces an increasing pressure drop between the high pressure chamber and the passage. Thus, the pressure P.sub.c in the passage decreases for increasing bypass flow. This same pressure is communicated through small passages to chambers which act against a face of the piston in a direction to reduce bypass flow. This force acts along with a spring force, the reaction force, and the force on a low pressure face of the piston caused by the low pressure side of the system to balance the pressure of the high pressure side of the system acting against a high pressure area of the piston. As bypass flow increases, P.sub.c decreases, reducing this counterbalancing force and allowing a given high pressure to open the bypass valve further than would be the case without this compensation.
In this way, the regulator of the '713 patent is designed to maintain a constant pressure across the metering valve. Even so, at high bypass flows, the pressure across the metering valve tends to be too small. This is due to the compensating force becoming too small at these flows. Thus, while the '713 patent gives a more constant pressure drop than previous regulators, that pressure drop is not constant for all bypass flows and pressures.